The present invention relates to switching voltage regulator circuits. More particularly, the present invention relates to step-down direct current-to-direct current (DC-to-DC) switching converter circuits (also called "buck switching regulators") that provide lower noise and programmable slew rate limiting compared to previously known buck switching regulators.
FIG. 1 illustrates a previously-known buck switching regulator that provides a predetermined and substantially constant output voltage V.sub.OUT from an unregulated positive supply voltage V.sub.IN (e.g., a battery) for driving a load R.sub.L, which, although simply shown as a resistor, may be, for example, a portable communication device or a computer. Buck switching regulator 10 includes controlled current source 12, transistors 14 and 16, diode 18, inductor 20, capacitor 22, and control circuit 24. Control circuit 24 generates control signal V.sub.r, which switches at a desired switching frequency, typically 100-300 kHz.
Regulator 10 operates as follows: at the beginning of a switching cycle, control signal V.sub.r goes HIGH, causing controlled current source 12 to conduct current I.sub.1, and turn ON transistor 14. The collector of transistor 14 drives the base of transistor 16, turning ON transistor 16. Once transistor 16 turns ON, the emitter current of transistor 16 rapidly increases at a high positive current slew rate. Following current slewing, the voltage at inductor input node V.sub.SW rapidly increases at a high positive voltage slew rate to approximately V.sub.IN. Diode 18 is OFF, and a voltage V.sub.L of approximately V.sub.IN -V.sub.OUT appears across inductor 20, which converts the switched voltage pulses into inductor current I.sub.L. Inductor 20 and capacitor 22 form a low-pass filter to remove components at the switching frequency and its harmonics from output voltage V.sub.OUT.
Control circuit 24 monitors output voltage V.sub.OUT and provides control signal V.sub.r that regulates the output voltage by varying the ON-OFF times of transistor 16 (i.e., varying the regulator's duty cycle, which is the percentage of time that a switch is ON during a cycle of operation). In particular, when V.sub.r is LOW, controlled current source 12 turns OFF, which turns OFF transistors 14 and 16. As transistor 16 turns OFF, the voltage at inductor input node V.sub.SW rapidly decreases at a high negative voltage slew rate to approximately GROUND. Following voltage slewing, the emitter current of transistor 16 rapidly decreases at a high negative current slew rate. In addition, V.sub.L changes to approximately -V.sub.OUT, diode 18 turns ON and conducts inductor current I.sub.L, and inductor input node V.sub.SW remains at approximately GROUND until the next cycle of control signal V.sub.r.
During each switching cycle, therefore, inductor input node V.sub.SW switches between approximately V.sub.IN and GROUND. To minimize instantaneous power loss and self-heating in transistor 16 and maximize overall efficiency during switching transitions, the positive and negative current and voltage slew rates of transistor 16 are relatively high. The fast slew rates, however, also generate electromagnetic interference (EMI), commonly called "noise," in the form of both conducted and radiated interference.
Some buck regulator applications require both low noise and high efficiency, such as data acquisition systems and communications systems. Although a linear regulator would satisfy the noise requirements for such applications, linear regulators cannot provide the required efficiency. Conventional buck switching regulators can meet the efficiency requirements for such applications but cannot satisfy the low noise requirements. It therefore would be desirable to provide buck switching regulator circuits that provide high efficiency but have limited voltage and current slew rates for low noise applications.